Solid propellant rocket motors typically comprise a rocket case, usually formed of metal or composite material, a thermal insulation layer lining the interior wall of the rocket case and a solid propellant. Positioned along the longitudinal axis and through the center of the solid propellant is a central bore leading to a nozzle in the aft section of the rocket motor. During ignition, the propellant bums and the combustion gases pass through the central bore and nozzle, thereby propelling the rocket.
The grain configuration and the rate at which the solid propellant bums establishes the thrust characteristics of that rocket motor. Unlike liquid propellant rockets, solid propellant rockets are unable to control or alter their thrust characteristics after ignition by adjusting the amount of propellants entering the area of combustion. However, the configuration of the propellant and the precise rate of propellant combustion and therefore, thrust characteristics of the solid rocket motor may be tailored to meet specific requirements by precisely controlling the surface area of the propellant exposed to ignition. To achieve such control, the propellant is provided with various passages and/or slots configured to yield the desired thrust characteristics. In many solid propellant rocket motors the configuration includes the aforementioned central bore as well as secondary slots formed radially and coaxially about the bore.
The central bore is usually formed in the rocket motor by positioning a mandrel in an empty rocket case substantially along the central longitudinal axis thereof. Uncured rocket propellant is then cast into the interior of the rocket case, filling the rocket case and surrounding the mandrel. After the rocket propellant is cured (solidified), the mandrel is removed from the rocket case leaving behind the central bore. However, several problems in fabricating the secondary slots have been encountered.
One approach which results in a very precise slot configuration is the machining of the slots into the propellant. This approach, however, is only convenient and cost effective when the rocket motor is large and the number of units being machined is small. When the solid rocket motor is small and there are many units to be manufactured, machining costs per unit can be prohibitively high. Additional problems associated with machining slots are that such operations produce undesirable quantities of propellant machining waste and the resulting slots must be radiographically inspected both before and after machining. Moreover, machining a solid propellant carries a danger of accidental ignition because of the heat associated with machining friction and accidental contact of cutting tools with the case.
A further difficulty with machining the secondary slots in the solid propellant is that there is little if any longitudinal stress relief upon thermal cool-down of the cured propellant. Shrinkage of the propellant as it cools at a rate different from that of any shrinkage of the case, causes relatively high stresses in the propellant along the longitudinal axis thereof. Such stresses which arise during cool down, can cause splitting or cracking of the propellant, thus rendering it unsuitable for the manufacturing operations.
An alternative approach to machining these secondary slots is to use polyurethane foam slot formers. Polyurethane foam can be cut inexpensively into small intricate pieces to use for small rocket motors. The polyurethane foam slot formers are attached to the mandrel and the solid propellant is molded or cast around the mandrel and the polyurethane slot formers. Although the removal of the mandrel from the solid propellant in this method creates the central bore, the polyurethane foam used to shape the slots, cannot be easily removed from the solid propellant because of the structural integrity of the polyurethane foam. Attempts to remove the polyurethane foam from the propellant can result in less than ideal separation, since some foam can remain attached to the propellant on the slot surface.
As a result of the difficulty of removing the polyurethane foam, solid propellant rocket motors using polyurethane foam are usually fired with the polyurethane foam slot formers still in place in the secondary slots. This can produce a pressure spike at the beginning of the ignition sequence because the flame front will cover the entire surface area of the solid propellant very quickly, but the polyurethane foam will hinder the exhaust gas of the burning propellant from escaping to the central bore and then to the exhaust nozzle. The pressure spike is higher than that of an empty machined slot of identical size and configuration and, if excessive, can cause catastrophic failure of the rocket motor. Another disadvantage of using polyurethane foam is the additional requirement of freon as a blowing agent for the foam.
Still another approach to forming the secondary slots in the solid rocket propellant is to attach the mandrel to an inflatable rubber tube in the desired form of the secondary slot. Secondary slots are formed when the solid propellant is poured around the inflated rubber tube and the mandrel. When the propellant is cured, the rubber tube is deflated, allowing the mandrel and the deflated rubber tube to be removed along the central bore. However, because of the limited dimensional stability inherent in such rubber tubes, reproducing either intricate shapes or exact slot dimensions is often difficult. The inflatable rubber tube is also undesirable because of the risk of rupture to the rubber tube which generally results in scrappage of the solid rocket motion. This is because of the failure of the forming operation.
Therefore, what is needed in the art is an inexpensive and reliable method of making secondary slots and removal of the formers in solid rocket propellants.